1. Field of the Invention
The present invention relates to a method for measuring the attenuation in digital transmission lines between a switching center and a subscriber station.
2. Description of the Related Art
Increasingly, telecommunications networks are changed over to broader band digital transmission methods. This also applies to the subscriber access area; consequently the telecommunications carriers are already implementing the changeover from analogue voice transmission to ISDN (Integrated Services Digital Network). At the same time, still further methods are being tested under the collective term of xDSL for Digital Subscriber Line (ADSL=Asymmetrical Digital Subscriber Line, HDSL=High bit rate Digital Subscriber Line, VHDSL=Very High bit rate Digital Subscriber Line) with considerably higher bit rates. All these methods suffer from a common problem in that the subscriber access lines in part were constructed many years ago, being originally intended only for transmission of the telephone voice channel. Since on the one hand these lines represents a huge investment and on the other hand it is known from investigations that these lines are also suitable for new transmission methods, albeit with certain restrictions, the network operators are endeavoring to use their existing line network as far as possible. However, this necessitates that prior to changing over, the access line for each subscriber be tested for suitability for the new services. This can happen in various ways. If in a particular case the data of the cable installed (length, characteristics of the cable) is known, then in most cases it can be worked out from this whether the line is suitable for the intended use. If this data is not known, then measuring becomes necessary, where among other characteristics the attenuation must also be measured; a very costly procedure. The reason for this is that usually attenuation measurement is carried out as an end-to-end measurement which requires two measuring instruments (level generator and level-measuring set), one at each end of the line. In addition, a measuring technician needs to go to the customer end of the line which can be very time consuming.
It is thus the object of the present invention to propose a method which considerably simplifies attenuation measurement when compared to the known method, and which can be carried out from one end of the line.
According to the invention, this object is met by the method of the present invention.
According to the invention a first pulse of a known frequency spectrum is sent from the switching center to the transmission line. From the multitude of the reflections received by the switching center in the time domain that pulse is selected which was caused by the total reflections at the end of the line of a certain subscriber station. Subsequently the frequency spectrum of the selected pulse is determined and from the frequency spectrum of the outgoing pulse and the selected received pulse, the attenuation of the transmission line is calculated, depending on its frequency. According to the invention, attenuation measurement is undertaken as a reflection measurement. This is possible if during measurement, total reflection occurs at the far end and if the line in operation is terminated at both ends with the same impedance. If the frequency spectrum of the pulse sent is precisely known, then, from the spectrum of the reflected pulse, the attenuation of the transmission line depending on the frequency can be calculated. The pulse sent travels along the line from the beginning to the end. In this, the amplitudes of its spectral lines diminish according to the line attenuation. At the end of the line the pulse is reflected, but due to the total reflection, the amplitudes of its spectral lines are not changed. During the subsequent return through the entire transmission line, the pulse experiences the same attenuation as during the outgoing travel. If for a particular frequency the relationship of the amplitudes of the respective spectral lines of the outgoing and returning pulse is formed, then the square of the line attenuation at this frequency is obtained. From this it is finally possible to calculate the sought attenuation value by calculating the square root. For practical realization this does however place very high demands on the dynamics of the measuring arrangement.
Attenuation is predominantly caused by line losses. To be sure, to a small extent reflections too, cause line losses which result in several pulses travelling to the line input. From these returning pulses, that pulse must be selected which was caused by the total reflection at the end of the line. This is possible if these pulses arrive at the line entry at various points in time, corresponding to their differing run-times. Individual reflection locations must however be spaced far apart so as to remain able to be distinguished at the respective pulse width. Ideally the right pulse is selected by an experienced measuring technician who can see the complete reflection image on screen. However, for the less experienced operator or for automatic measurements, evaluation can also take place entirely by a control computer.
In the case of low frequencies, the wave impedance of the line is complex. During reflection in this complex impedance, the phase angle is rotated to a different extent with different frequencies and as a result the pulse is considerably widened. A particularly interfering reflection occurs at the beginning of the line because the respective pulse is not attenuated by the transmission line. This pulse becomes so wide that it covers the significantly smaller pulse from the reflection at the end of the transmission line. In order to counteract this, according to a preferred embodiment, the frequency range of the pulse is shifted to higher frequencies, if possible far enough for the wave impedance of the line in this frequency range to be almost real. This shift is tantamount to modulation of a carrier with the pulse. This method is limited in that on the one hand line attenuation increases as the frequency increases so that the useful signal is gradually diminishing; and on the other hand in that the frequency at which attenuation is to be measured must also be within the spectrum of the pulse.
According to this preferred embodiment, the first pulse is sent through the transmission line in the shape of individual sine-shaped signals whose frequency has previously been determined from the frequency spectrum of the first pulse, and the reflections in the time domain are determined by inverse Fourier transformation of the frequency spectrum of these reflections. In this way the pulse spectrum can be selected as freely as possible and in spite of a low signal voltage, a very large pulse can be obtained. The spectral lines of the pulse are transmitted in sequence in the shape of sine-shaped permanent signals, and the pulse is generated mathematically by inverse Fourier transformation of its frequency spectrum in the frequency range. The pulse shape is determined by the choice of window function. Although the individual frequencies are applied to the line only at voltages of a few volts, the voltage of the total pulse is for example 100 V. This method is basically known as frequency range reflectometer; it is largely unsusceptible to pulse-shaped interferences on the line.
In contrast to the classical attenuation measurement in the frequency range, in which at the end of the line the superimposition of the main wave and all further waves generated by multiple reflection, is measured; in the case of measurement with the pulse reflection method only the main wave is acquired. Furthermore, it is not the double attenuation (xe2x80x9cdoublexe2x80x9d in the sense of a logarithmic attenuation constant) of the transmission line that is measured but the attenuation of a transmission line of the same type, which is twice as long. Due to the attenuation portion caused by mismatch, which is thus independent of the line length, a further measuring error results. However, this error is relatively small.
According to a further embodiment of the method, when using a known measuring setup and when specifying exactly defined frequency relationships, an exact attenuation measurement for the application range is made possible. In this, during selective reflection measurement in the frequency range, additionally the pulse in the frequency range is shifted to higher frequencies in order to reduce pulse widening by dispersion.
The method according to the invention thus makes it possible to qualify subscriber connection lines for new services (ISDN, ADSL, etc.) by measuring the attenuation from the switching center. This results in considerable time savings in the case of extended lines; in the case of star-shaped networks, measuring can be automated with only one measuring instrument and a selector switch. Consequently, this method for measuring the attenuation can be carried out at considerably lower costs. It is sufficiently accurate for the intended field of application, but it is inaccurate in the case of lines which are extremely low in attenuation and in addition highly mismatched.